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I'm sorry, did you change your thermostat? If not, I think yours is broken open. That's 75 degrees C, and op temp should be 88-96 C or thereabouts.

If that temp was on the highway, I think therm is broken open even more so...and I don't see how a different ET cap would affect temperature...and if it did, I'm pretty sure that's not as good a thing as you think it is.

When therm breaks open, first sign, besides seeing the needle drop below dead center, is that the mpgs go down, by maybe 2-3 mpgs, since you're not at op temp and the engine isn't working efficiently.

Just did a quick calculation. Apparently 1 bar cap is good enough. Take it for what it's worth:

Assuming 96 degree Celcius coolant temperature, which is pretty much the max, the pressure of an ideal gas (e.g. air) is 1.3516 atms. That means you'd need a cap capable of holding 1.3516 bars at the minimum. This would be called a 0.3516 bar cap. This would, however, increase the boiling point of the coolant by a mere 8.5 degrees (Celcius):

So %100 water as your coolant would boil at 108.5 degrees Celcius instead of 100 degrees.
%50-%50 mix would boil at 114.5 degrees Celcius instead of 106.

These numbers are ok theoretically, but they are too close for comfort in real life. Locally, temperatures could be higher than these (e.g. engine block), and your coolant will turn into gas whenever this occurs. Since gas occupies more space than liquid, you'd be replacing your hoses quite often. I am assuming the engine block can take a beating.

On the other hand, 1 bar cap increases the boiling point by about 24 degrees Celcius. So for 100% water, you'll get 124 degrees Celcius as your new boiling point. For 50-50 mix, it will be 130 degrees. Similarly, 2 bar cap raises it by 48 degrees, so your new boiling points for 100% water and 50-50 mix will be 148 and 154 degrees, respectively.

Since the operating temperature of your engine doesn't change, using a 1 bar cap will lower your chances of blowing the head gasket. You'll probably see your car smoking before the temp gauge hits the maximum. But there is more chance of introducing excess air (gas) into the system, which could require you to use heavy duty hoses in lieu of what comes from the factory.

There's more to it though. The n54 electric water pump is variable speed and runs after the engine is shut off.

Point was the n54 cooling system operates a more traditional 1 bar cooling system.

I agree there's more to it and I doubt we'd ever get to the bottom of the mystery of the M54's 2bar cap. The discussion is healthy though and I thank you for bringing it up.

On the above quote, my point wasn't that the N54's pump doesn't vary in speed, but rather that at full rev the M54 water pump has no choice but to rev with the motor. At a 5.5" crank and a 5.185" stock pump pulley, the motor's 7000 rpm translates to around 7400 at the pump. I suspect BMW upped the pressure to alleviate some of the cavitation similar to an outboard when you give it too much revs.

The N54 water pump may have a variable-speed pump, but I doubt it gets to those high revolutions. As "proof" I submit our own daily drivers. Our water pumps cool our cars just fine at a paltry 750 rpm even in a 120-degree ambient, heat-soaked engine.

Local pressures is not the issue here. The pressure can drop locally below 1 atm no problem, but it's integral over the entire system can never drop below 1 atm (in a closed system of course, due to conservation). It if drops below 1 atm by x amount across the impeller, it should increase by the same x amount somewhere else.

Regardless, we are not talking about the pressure at the impeller or how much it can drop during operation. Question is why would a 1 bar cap would allow a higher pressure gradient across the impeller than a 2-bar cap? If the cavitation is a concern with a 1-bar cap, it could very well be a concern with a 2-bar cap. In fact, more of a concern with the 2-bar cap than the 1-bar cap. 2-bar cap has a greater tolerance than a 1-bar cap, hence by conservation, could allow even lower pressures to be attained across the impeller.

Edit: I was thinking about it (I misse Sean's earlier post explaining the same realization)- and there's no way the temperatures increase enough to bring the system pressure to 2 bar (let alone 3 bar) if the volume stays constant. In a closed loop (no venting), going from 25°C to 95°C would increase the pressure by like .2 atm (from 1 atm). So the 2 bar cap must be there mostly to alleviate pressure in the instance that the coolant level rises too high (say when the heater core closes). So I think a 1 bar cap would just make it even safer.

Agreed, if something drops by 1atm at one spot, then it should increase by 1atm somewhere else. But there are properties dependent on absolute pressure. If you have localized regions where the pressure is lower than the vapor pressure of the coolant at a given temperature, you will run into cavitation. At higher system pressures (integrated pressure being overall 3atm or whatever) it is unlikely there would be even localized regions where the pressure drops low enough, since the difference stays constant.

Hypothetically speaking, let's say that at one point in the loop, pressure drops by 0.8atm. If the system pressure overall is 1atm, then that one point will be at 0.2 - quite possibly a coolant mixture would form bubbles at that pressure. If the system pressure was 3atm, then that one point will be at 2.2 - still well above atmospheric pressure.

So the question isn't about a pressure gradient - the cap will not effect the difference in pressures for any given points. But it will effect absolute pressure throughout the whole loop, and that is worth considering.

We know the boiling point isn't an issue. Even running straight water, the bp should be high enough at 2atm to not be an issue at normal operating temperatures. Cars like the 335 are proof, especially since they run similar coolant temperatures. So then we must consider other reasons BMW elected to use a 2 bar cap instead of a 1 or even 1.4 bar cap. Cavitation is a good theory - at red line and a high enough operating temperature, the difference in vapor pressure might just be too close to the pressure where the coolant is flowing the fastest (around the water pump's impeller)

Just did a quick calculation. Apparently 1 bar cap is good enough. Take it for what it's worth:

Assuming 96 degree Celcius coolant temperature, which is pretty much the max, the pressure of an ideal gas (e.g. air) is 1.3516 atms.

I don't know where you got this number. Did you assume a 1 atm starting point at 25C for this and calculate the change in pressure given the same volume but higher temperature?

I mean did you do: P1/T1 = P2/T2

Anyway, we don't really have numbers to work with here. We need volumetric flow rates, temperatures at various points in the system, pressures at various points, and some system specs (total volumes of water and air/steam). It is more complex than doing some ideal gas calculations. You need data. I designed a model of a presurized water reactor for my senior design project. This cooling system uses all of the same fundamentals. To get the data we need will require at least $1000 in instrumentation, plus the time and know how to hook it up.

My opinion is that the system does not operate anywhere near its 3 bar absolute maximum. If it did, then that cap would be constantly releasing steam/air. That limit is only there for emergency pressure relief. It will release steam, not air (just a little from the ET), when triggered because to reach 3 bar the system must boil some water into steam. My thought is that the cap is there only to relieve pressure to save the engine's seals in the event of an over heat. The system likely runs just above 1 atm in normal operation due to the vapor that forms in the ET, but that is it. The numbers we have suggest this. You can't pressurize a system to 3 bar with 100C water/antifreeze (unless it's its a solid system), that's just basic physics.

Also, a note on pressure units: When the cap says it is a 1 atm or a 2 atm cap, it means 1 atm or 2 atm gauge. Gauge pressure is your absolute pressure minus your barometric pressure (1 atm, 14.7 psi, the air you breath). That's just a note for those reading, who may be confused by the term's usage.

Edit: I was thinking about it (I misse Sean's earlier post explaining the same realization)- and there's no way the temperatures increase enough to bring the system pressure to 2 bar (let alone 3 bar) if the volume stays constant. In a closed loop (no venting), going from 25°C to 95°C would increase the pressure by like .2 atm (from 1 atm). So the 2 bar cap must be there mostly to alleviate pressure in the instance that the coolant level rises too high (say when the heater core closes). So I think a 1 bar cap would just make it even safer.

Haha we came to the same conclusion.

Also, another conclusion from my previous conclusion. A 1 bar cap isn't going to do anything in day-to-day operation of your car. If it overheats, though, the engine seals will be less likely to fail (this is a very good thing).

Also, another conclusion from my previous conclusion. A 1 bar cap isn't going to do anything in day-to-day operation of your car. If it overheats, though, the engine seals will be less likely to fail (this is a very good thing).

Thinking off the top of my head here...pv=nrt would probably only work for a static system. The entire cooling system has a pressure differential (consider the points directly before and directly after the water pump) which causes flow. Also, the volume of the system is fixed, and the volume of a given amount of air changes depending on pressure, no?

Regardless of the science behind it, I agree with TxZHP04 in post #13; all this modification will do is introduce a leak as the "mean system pressure" in the ET exceeds the limits of the lower cap. Changing the cap will not change the operating pressure of the system, as it simply bleeds excessive pressure (which only happens if the T gets too high, right?). Think more like underdrive pulley if you want to reduce the pressure...and have a head gasket handy.

Thinking off the top of my head here...pv=nrt would probably only work for a static system. The entire cooling system has a pressure differential (consider the points directly before and directly after the water pump) which causes flow. Also, the volume of the system is fixed, and the volume of a given amount of air changes depending on pressure, no?

Regardless of the science behind it, I agree with TxZHP04 in post #13; all this modification will do is introduce a leak as the "mean system pressure" in the ET exceeds the limits of the lower cap. Changing the cap will not change the operating pressure of the system, as it simply bleeds excessive pressure (which only happens if the T gets too high, right?). Think more like underdrive pulley if you want to reduce the pressure...and have a head gasket handy.

Long story short: waste of time.

It's not a waste of time and there is a very simple justification for why ideal gas law should work here.

First of all, we are only interested in the pressure that the gas exerts onto the cap in the expansion tank. This gas doesn't circulate the system. Coolant does. So it doesn't have anything to do with the system being static.

Also, volume of air is fixed if it is contained, as in the case of ET (since the coolant expands a little, the volume of the gas actually decreases by the same amount, but this is negligible). In a system with a fixed volume, increasing temperature increases the Pressure only. Directly seen from the ideal gas law.

Second, while the coolant reaches temperatures above the 96 degree celcius in the engine block, it is around 96 degree celcius in your lower radiator hose. It is likely to be higher by 10-20 degrees in the ET, but that doesn't affect the conclusion (10-20 degrees more will increase the pressure by a fraction of a bar).

At thermal equilibrium (e.g. at operating temperature), the coolant and the gas in the ET have the same temperature.

I will agree, but only to the point that I can't refute your logic. Just so I am clear, you believe the overall pressure in the system is somehow determined or limited by the radiator cap itself? You mention that a "2 bar cap would raise the boiling point xx degrees."

I will agree, but only to the point that I can't refute your logic. Just so I am clear, you believe the overall pressure in the system is somehow determined or limited by the radiator cap itself? You mention that a "2 bar cap would raise the boiling point xx degrees."

Overall pressure is determined not by the cap, it is determined only by temperature of the gas (in other words, temperature of the coolant in the ET at thermal equilibrium). Since fluids are incompressible, the gas pressure is transferred throughout the system. There will, of course, be pressure differences at different parts of the system, but the average pressure is the pressure of the gas.

Cap pressure rating important, say, when you're tracking the car. In situations where you are driving above 4k rpm in 2nd-3rd-4th gears all the time, your radiator cannot keep up with the cooling requirement of your engine alone. So temperature of your engine, hence that of the coolant increases (your gauge doesn't show this, but this is what happens). Temperature increase results in pressure increases. Gas molecules start beating on the cap more and more. But since the cap can take it, overall effect is just an increase in pressure. It is not the cap that increases the boiling point per se, but the pressure of the gas that does so.

A simple example is the following: Pressure cookers cook the food faster by allowing pressure build up inside. This increases the boiling point of the water. Result is your beans cook faster at a temperature higher than 100 degrees.

For the record, I was referring to 1bar caps being a waste of time. I'm enjoying this discussion.

Quote:

Originally Posted by SeanC

Overall pressure is determined not by the cap, it is determined only by temperature of the gas (in other words, temperature of the coolant in the ET at thermal equilibrium). Since fluids are incompressible, the gas pressure is transferred throughout the system. There will, of course, be pressure differences at different parts of the system, but the average pressure is the pressure of the gas.

I agree with this.

Quote:

Cap pressure rating important, say, when you're tracking the car. In situations where you are driving above 4k rpm in 2nd-3rd-4th gears all the time, your radiator cannot keep up with the cooling requirement of your engine alone. So temperature of your engine, hence that of the coolant increases (your gauge doesn't show this, but this is what happens). Temperature increase results in pressure increases. Gas molecules start beating on the cap more and more. But since the cap can take it, overall effect is just an increase in pressure. It is not the cap that increases the boiling point per se, but the pressure of the gas that does so.

I follow what you are saying, but I disagree with your reasoning. The radiator can keep up with the cooling requirement, or all track cars would eventually overheat (thermal runaway or whatever the proper term is). The engine heats up, thus heating the coolant. Then, then thermostat opens more to regulate the temperature at a fixed setting by varying the flow through the radiator. The cap is basically just a safety valve, to prevent parts from blowing apart or failing altogether in the event of an over-pressure situation (overheated engine). If you were for some reason in a situation where the radiator couldn't keep up and the pressure continued to rise, sure a better cap would help a little, but that's pretty much just a band-aid. By your logic, wouldn't a system with no vent would have the highest boiling point, and as such cool the "best?"

I do not think that the M54 operates below 1 bar in all running conditions, and I still don't understand how putting on a lower-pressure cap would help anything. All it will do is cause a coolant leak from cap venting when there is no need for the venting in the first place.